Nepal Plane Crash Caused By Bad Weather, Says Preliminary Probe; Read How Air Accidents Are Probed

The Nepali passenger plane carrying 22 people crashed in Western Nepal on Monday because of bad weather, according to a priliminary investigation by the Civil Aviation Authority of Nepal (CAAN).

Twenty bodies have been recovered from the site of crash in Western Nepal's Mustang district, while the search for remaining two people on board is on. 

The Tara Air's plane was carrying 22 people, which included four Indians, two Germans and 13 Nepali passengers and a three-member Nepali crew.

The Canadian-built turboprop Twin Otter 9N-AET plane went missing on Sunday in the mountainous region of Western Nepal within 15 minutes of take-off from the popular tourist destination of Pokhara. Its last contact with the ground control was from the sky above Ghorepani, which is on the air route to Jomsom where the plane was headed.  

The wreckage of the plane was spotted on Sunday and was physically located on Monday. The Nepali government has formed a five-member commission of inquiry headed by senior aeronautical engineer Ratish Chandra Lal Suman to find out the cause of the Tara Air plane crash, officials said. 

Preliminary investigation revealed that the aircraft had crashed into the mountains after it swerved to the right, instead of turning to the left due to bad weather, CAAN Director-General Pradeep Adhikari said during a meeting of the International Committee of the Parliament on Monday. 

Here we explain how an aircraft accident is investigated, what are the investigation's various aspects, and what's the usual timeline. 

How does an investigation start?

The aircraft accident investigation starts by securing the site of the accident and collecting as much physical evidence as possible. The recovery of the 'block box' is the priority. 

Investigators take photographs and videos of the site, the aircraft, the debris, and properly store all debris and big parts of the plane that are salvaged. 

"They also conduct interviews with eyewitnesses and draw charts showing the debris field and any indications of how the aircraft hit the ground, such as the angle of impact, the distribution of debris and other details," according to Daniel Kwasi Adjekum, an Assistant Professor of Aviation at the University of North Dakota.

Daniel added that investigators also collect all the documents related to the plane, its crew and its recent flights for forensic analysis.

The meaning, importance of 'black box'

What's commonly called the 'black box' in most aircraft are usually two separate devices – a cockpit voice recorder (CVR) and a flight data recorder (FDR). Recovering these devices is the first priority for investigators. 

Several microphones are installed in a cockpit that capture all the sounds there, which ranges from conversations to even faint clicking of switches and beeps and alarms from the various sensors and panels installed there. All of this sound is stored in CVR. This is important in as it provides cockpit crew's voices, engine sounds, warnings from instruments, and every other piece of audio.

FDR stores all of the plane's operating data. There are several parameters that are tracked by FDR through the various sensors installed in the plane, such as the time, altitude, speed, fuel, etc. 

Together, the data from CVRs and FDRs is critical in the reconstruction of a timeline of the flight. It can provide clues from either the data or sound from the cockpit about the cause of the accident, such as whether pilot ignored warnings or sensors reported an anamoly that caused the accident. 

Investigators can also create video simulations of the aircraft’s trajectory from such data.

Diverse roles of investigators

There are specific technical support and investigation teams that look at technical aspects of the accident, such as the air traffic control activity and instructions, weather at the time of the accident, human issues like crew experience and training, plane’s maintenance records, emergency response, safety equipment, and the aircraft’s performance and sub-systems.

"They may disassemble the crashed plane’s engines or other components and use flight simulators to attempt to experience what the pilots were dealing with," writes Daniel, quoted above in the story. 

Forensic experts are also involved in the investigation as they test the dead for identification purposes. Moreover, they test survivors among the passengers as well as the crew for drugs or other substances to understand whether their conduct was compromised in any way or whether they were inebriated or in any kind of irregular mental or physical state that could have led to the accident.

Timeline of air accident investigation

The aircraft accident investigators must release a preliminary report to the International Civil Aviation Organization in 30 days, which is the specialised agency of the United Nations for civil aviation. 

The final report is usually expected to follow the preliminary report before the accident's first anniversary. 

In case of accidents where a final report cannot be released in the year's time, investigators release important updates or an interim report on the first or subsequent anniversaries of the accident detailing the progress made so far.

Crucially, the objective of the aircraft accident investigation is to not assign blame, but to find what caused the accident – human error, plane's design flaw, extreme weather, bird hit, etc – and to work on it to prevent a similar unfortunate event in future. 

(With PTI inputs)